Polymorphisms in the 5-HTTLPR gene mediate storage capacity of visual working memory

Abstract

Working memory (WM) is a limited capacity system that permeates nearly all levels of cognition, ranging from perceptual awareness to intelligence. Through behavioral, electrophysiological, and neuroimaging work, substantial gains have been made in understanding this capacity-limited system. In the current work, we examined genetic contributions to the storage capacity of WM. Multiple studies have demonstrated a link between the serotonin transporter-linked polymorphic region (5-HTTLPR) and cognition, where better performance is observed in individuals possessing a copy of the short (s) variant of the polymorphism compared with individuals homozygous for the long (l) variant. We predicted the same profile in WM performance, such that estimated capacities of l/l carriers should be smaller than s/s and s/l carriers. To measure WM capacity, we implemented a change detection task, which requires observers to actively maintain the color and spatial location of briefly presented squares over a short retention interval. In line with our prediction, we observed similar WM performance between s/s and s/l groups, and these individuals performed better than the l/l group. We then discuss the distribution of the serotonin transporter system and parallels between WM and attention to provide insight into how variation in the 5-HTT polymorphism could lead to individual differences in the storage capacity of WM.

Figure 1

Change detection task. Participants maintained fixation and were instructed to remember the colors of all objects presented on the display. Set sizes used were 2, 4, 6, and 8. After a short delay period, participants were presented with a probe item that could either be the same or different color with respect to the memory item presented in the same spatial location. Participants responded with one of two buttons to indicate whether it was a same or change trial.

Figure 2

(A) Estimated visual WM capacities as a function of the presence or absence of the short allele. Performance between short carriers (s/s, s/l) and homozygous long carriers (l/l) was statistically significant (p < .05). (B) Estimated visual WM capacities as a function of 5-HTT genotype. Performance between s/s and s/l groups was statistically indistinguishable (p > .9). Compared with carriers of the short allele, individuals homozygous for the long allele (l/l) performed worse in the change detection task, as demonstrated by a lower visual WM capacity estimate (p < .05).